1. Field of the Invention
The present invention relates, generally, to electromagnetic actuators and, more specifically, to a method of controlling an electromagnetic actuator of an internal combustion engine.
2. Description of the Related Art
A direct-injection system includes a plurality of injectors, a common rail, which feeds pressurized fuel to the injectors, a high-pressure pump, which feeds fuel to the common rail through a high-pressure feed line and is provided with a flow-rate adjusting device, and a control unit, which controls the flow-rate adjusting device so as to cause the fuel pressure on the inside of the common rail to be equal to a desired value, which normally varies in time as a function of the engine operating conditions.
A high pressure fuel pump, as described in patent application EP2236809A1, includes a pumping chamber, in which a piston slides back and forth, an intake pipe regulated by an intake valve to feed low-pressure fuel to the pumping chamber, and a delivery pipe regulated by a delivery valve to feed high-pressure fuel from the pumping chamber along the feed line to the common rail.
The intake valve is normally pressure-controlled and, in the absence of external intervention, is closed when the fuel pressure in the pumping chamber is higher than the fuel pressure in intake channel, and is open when the fuel pressure in the pumping chamber is lower than the fuel pressure in intake channel. The flow-rate adjusting device is mechanically coupled to the intake valve so that, when necessary, the intake valve can be kept open during the piston pumping phase, thus allowing the fuel to flow out of the pumping chamber through the intake channel. In particular, the flow-rate adjusting device includes a control rod, which is coupled to the intake valve and is movable between a passive position, in which it allows the intake valve to close, and an active position, in which it prevents the intake valve from closing. The flow-rate adjusting device includes, furthermore, an electromagnetic actuator, which is coupled to the control rod so as to move it between the active position and the passive position. The electromagnetic actuator includes a spring, which holds the control rod in the active position, and an electromagnet, which is designed to move the control rod to the passive position by magnetically attracting a ferromagnetic anchor, which is integral to the control rod, against a fixed magnetic armature.
In use, the high-pressure pump described in patent application EP2236809A1 produces a sound similar to a ticking noise, which can clearly be perceived when the engine runs slow (namely, when the overall noise produced by the engine is moderate). The noise generated by the high-pressure fuel pump can be perceived in a clear manner also because the high-pressure fuel pump, having to receive the motion from the drive shaft, is directly mounted on the head of the engine, the head of the engine transmitting and diffusing the vibration generated by the high-pressure pump.
The noise produced by the high-pressure pump in use is basically due to the cyclical hits of the mobile equipment of the flow-rate adjusting device (namely, of the control rod and of the anchor) against the intake valve (strike corresponding to the active position) and against the magnetic armature of the electromagnet (strike corresponding to the passive position).
In order to reduce this noise, one could act, via software, upon the intensity and the waveform of the control current of the electromagnet, so as to minimize of kinetic energy of the mobile equipment when it hits the intake valve and the magnetic armature. Experiments have shown that, by acting via software upon the control current of the electromagnet, one can significantly reduce the kinetic energy of the mobile equipment when it hits the magnetic armature. On the other hand, experiments have shown that, by acting via software upon the control current of the electromagnet, it is much more complicated and expensive to significantly reduce the kinetic energy of the mobile equipment when it hits the intake valve.
In order to significantly reduce the kinetic energy of the mobile equipment at the moment of the impact, the control system should excite the electromagnet with a control current that is as close as possible to the “limit” control current (which gives to the mobile equipment the “minimum” kinetic energy at the moment of the impact), but, especially, the control system should excite the electromagnet with a control current that is never below the “limit” control current, otherwise the actuation is lost (namely, the mobile equipment never reaches the desired position due to an insufficient kinetic energy). The value of the “limit” control current is extremely variable from case to case due to constructive losses and to creeps caused by times, temperature, battery voltage, engine speed and, usually, different operating point.
In order to reduce the noise produced at the moment of the impact of the mobile equipment against the intake valve, since there is no way to check whether the limit position has been reached (namely, whether the actuation has been completed), one can advantageously use an electromagnetic actuator provided with a one-way hydraulic brake, which is integral to the control rod and slows down the movement of the rod; in particular, the hydraulic brake moves the control rod between a passive position, in which the control rod allows the intake valve to close, and an active position, in which the control rod does not allow the intake valve to close; and the hydraulic brake is suited to generate a high braking force, when the control rod move towards the active position, and to generate a negligible breaking force, when the control rod moves towards the passive position.
When the mobile equipment hits the magnetic armature, the control system is able to check whether the limit position has been reached (namely, whether the actuation has been completed) by observing the fuel pressure in the common rail (when the control rod hits the magnetic armature, the intake valve closes and, therefore, the high-pressure fuel pump starts to pump pressurized fuel, which increases the fuel pressure in the common rail). Then the control system can progressively reduce the control current, until the reaching of the limit position (namely, the completion of the actuation) disappears; now it can slightly increase the control current so as to carry out the actuation with the “minimum” kinetic energy at the moment of the impact.
Over the course of time, though, the control system proves to be inefficient in the limitation of the kinetic energy of the impact and, therefore, in the limitation of the noise produced, due to behaviour losses of the magnetic actuator.